Process for making gaseous discharge tube



July 23, 1957 R. H. MITCHEL pnocsss FOR MAKING GASEOUS DISCHARGE TUBEFiled Dec. 30. 1954 32% ZEk/ wl w ATTORNEY Unite States ate'nt 2,800,383Patented July 23, 1 957 Ralph H. Mitchel, Flint, Micln, assignor toGeneral Motors Corporation,Detroialviicln, a corporation of DelawareApplication December 30, 1954, Serial No. 478,556

11 Claims. (Cl. 31622) This invention relates to a process formanufacturing gaseous discharge tubes generally of the type shown inUnited States Patent 2,478,119, granted August 2, 1949, to Ralph H.Mitchel. Such gaseous discharge tubes, which find utility in highfrequency ignition systems as well as in spark plug testing devices andthe like, consist essentially of two electrodes having surfacesspacially separated to form a spark discharge gap and supported within aglass or similar envelope containing an inert gas atmosphere. The tubesare designed to discharge at a specific voltage, the features of thetube significant in this respect being the spacing and composition ofthe electrodes and the composition and pressure of the inert gas.

It has been found by experience that the presence of even minuteimpurities such as oxides within the envelope have a substantial effectnot only upon the initial electrical characteristics of the tube, suchas the voltage required to cause discharge, but also upon the rate atwhich these characteristics vary with continued operation. Thus, it isessential that such impurities be completely removed during manufacture.Also, it has been found highly advantageous, if not essential, that theelectrode surfaces be quite dense and smooth as well as of very accuratedimensions. With tubes lacking these important features, the electricalcharacteristics vary substantially over the life of the tube. Forexample, with even very small amounts of oxide impurity present on theelectrodes and with electrode surfaces which are relatively porous, thevoltage required to cause discharge will gradually increase withoperation, thus rendering the tube useless.

if is an object of the present invention to provide a process formanufacturing tubes, of the type referred to above, which will greatlyincrease their uniformity of performance and durability. Morespecifically, it is an object of the present invention to provide aprocess for manufacturing such tubes which will assure the removal ofimpurities from the surfaces of the metal parts such as the electrodesand electrode supports and which will further assure dense, smootheelectrode surfaces of accurate dimensions.

Other objects and advantages of the invention will appear more clearlyfrom the following description of preferred embodiments and from thedrawings in which:

Figure 1 is a view partially in vertical section and partially inelevation of one form of discharge tube manufactured in accordance withthe present invention;

Figure 2 is an exploded view of the tube showing the various componentparts; and

Figure 3 is an end view of the tube shown in Figure 1 during one step inits manufacture and prior to scaling the exhaust stems. I

Referring now to Figure 1, there is shown a tube illustrative of thetype to which this invention relates and comprising two cupped endmembers 4 and 6, provided with out-turned annular flanges 8 and 10respectively whlch are bonded to glass member 12 to form a gas sealedenvelope therewith. Electrical contact members 9 and 13 are welded orotherwise secured to the concave exteriors of the end members as shown.The end members 4 and 6 are made of an alloy having substantially thesame coefiicient of expansion as the glass to which it is bonded so thatthe structure is durable and resistant to cracking over a widetemperature range. Typical of such alloys is that consisting of 29%nickel, 17% cobalt, 3% manganese, and the balance iron, and referred toin the trade as Kovar. Other metals with matching glass may, of course,be used. Welded or otherwise secured to the inner convex surfaces of endmembers 4 and 6 are discshaped electrodes 14 and 16 respectively, theopposing fiat surfaces lift and 20 of these electrodes being in spacedrelation with respect to each other to provide a spark discharge gap.The disc-shaped electrodes are preferably made of a fused or sinteredmixtured of finely pulverized metal such as iron and a salt or oxide ofa metal of low work function such as barium oxide. To provide uniformdischarge characteristics irrespective of the amount of light to whichthe tube is exposed, the interior surfaces of the tube and especiallythe surface of the glass envelope may be coated with a small amount ofradioactive substance such as radium bromide. The envelope is filledwith a inert gas of predetermined composition and under predeterminedpressure to impart the desired discharge characteristics. For furtherand more detailed description of such tubes and various embodiments andmodifications thereof, reference may be made to the above-mentioned U.S. Letters Patent No. 2,478,119.

As pointed out above, it is highly desirable for the durability anduniformity of operation of such a tube that the interior of the envelopebe freed of impurities such as oxides and the like and that the opposingflat electrode surfaces 18 and 20 be smooth, dense, and of accuratedimensions. Heretofore it has been the practice to remove impuritiesduring manufacture of the tube by flowing hydrogen through the envelopewhile the metal and glass parts are heated. This method has been foundto be unsatisfactory for the reason that in removing the oxygenimpurities from the electrode surfaces 18 and 20, a somewhat porous,spongy surface results, thereby greatly reducing the desirableproperties such as uniformity of electrical characteristics. Presumably,such a spongy surface is subject to considerable wear and sputteringfrom the force of the discharges and since the required dischargevoltage is a function of the closest electrode spacing, such wearresults in non-uniformity and eventually an increase in the breakdownvoltage. Also, the sputtered material collects on the interior surfacesof the tube, especially the glass envelope, resulting in a reduction ofthe electrical resistance across the gap to an unsatisfactory low value.

In accordance with the present invention, the oxides and otherimpurities are removed by first treating the interior of the envelopechemically, that is, with an acid or other liquid solvent material whichdissolves or loosens the metal oxides that may be present on thesurfaces of the electrodes and other metal parts. Subsequently theinterior surfaces are treated with hydrogen to remove the very lasttraces of impurity. Thus, in practicing the invention, there is firstadmitted to the interior of the envelope a quantity of acid or the likewhich is capable of dissolving or loosening the metal oxide and otherimpurities which may be present, removing the acid along with theloosened and dissolved oxides and other impurities, and then introducinghydrogen into the envelope and heating the electrodes in the hydrogen tothereby remove the lesser amounts of impurity. To assure the removal ofany impurity which may be introduced during other processing steps, itis highly advantageous to treat with the hydrogen only after such otherprocessing steps are completed and just prior to filling the envelopewith inert gas and sealing.

By the term chemical solvent as used herein is meant a liquid which iscapable of dissolving the oxides and other impurities at least to theextent necessary to loosen them from the interior surfaces. Thus, theinvention contemplates the use of liquids which will loosen the coatingof oxide and other impurities on the interior surfaces to cause them toflake off and drain out of the tube when the liquid is removed.

The following is a description of a preferred process embodying theinvention, reference being made to Figures 2 and 3.

Figure 2 shows the various component parts used in manufacturing thetube. The glass member 12 which is substantially cylindrical in shape ispreferably provided with two exhaust tubes 22 and 24 (shown afterscaling in Figure 2) in order to facilitate the hereinafter describedprocessing steps and in particular the step for chemically removingimpurities. End members 4 and 6 may be formed in any well-known mannersuch, for example, as by progressive drawing of blanks cut from stripstock. If these end members are of 29% nickel, 17% cobalt, .3%manganese, and the balance iron or a similar alloy, it is desirable toanneal them in hydrogen at a temperature of about 1650 F. for about onehour before sealing the glass member 12 thereto. Flanges 8 and 10 may bemachined to accurate dimensions in a lathe collet and should bethoroughly cleaned prior to the hydrogen annealing at 1650* F.

In a particular tube shown in the drawings, the electrodes 14 and 16 areof a sintered mixture of about 91% ion and 9% barium oxide. Theprocesses for making such sintered structures are, of course, well-knownin the art, and the precise method used forms no part of the presentinvention.

Electrodes 14 and 16 are bonded as by welding to the convex interiorsurfaces of end members 4 and 6 respectively and electrical contactmembers 9 and 13 welded or otherwise bonded to the concave exteriorsurfaces. Then flanges 8 and 10 are cleaned, hydrogen annealed, andjoined to the edges of the glass member 12, all as shown in Figure 3.The latter joining operation may be accomplished by heating the flangesand the glass member until the edge portions of the glass sofetn andthen bringing the metal and glass into contact to form the bond. Thecomposition of the glass is, of course, such as to provide a coefficientof expansion which substantially matches that of the flanges so as toprevent strains and cracking over a wide temperature range and also suchas to provide a good bond with the flange metal.

During these various processing steps, most of which require theapplication of heat, a considerable layer of metal oxide is formed onthe metal surfaces of the electrodes and end members. In accordance withthe present invention, the great bulk of this oxide is first removed bya chemical cleaning operation. The following procedure is illustrative:

First, one of the two exhaust stems (shown prior to sealing in Fig. 3)is plugged and there is admitted to the envelope through the otherexhaust stem, a quantity of acid or similar oxide solvent at leastsufiicient to dissolve or loosen the oxide and other impurity present.The solvent is left in the tube and agitated continuously until theoxide layer is removed and then is allowed to drain by removing the plugfrom the exhaust stem. For manufacturing on a small scale, a hypodermicsyringe and needle may be used to introduce the chemical solvent intothe envelope, agitation being produced by alternately squirting andsucking the solvent with said syringe. Alternatively, suflicient solventto only about half fill the envelope may be added and then mechanicallyshaken to cause the agitation required to thoroughly remove all theoxide impurity. For large scale production operations, it may bedesirable to flow the chemical solvent more or less rapidly through theenvelope by using one of the exhaust stems as an entrance and the otheras an exit,

2,soo,asa A 4.

the necessary agitation being provided by the movement of the solventpast the metal surfaces.

For chemically cleaning the type tube described herein, i. e., onehaving sintered iron electrodes and other metal parts of iron alloys, apreferred oxide solvent consists of a solution of hydrochloric acidcontaining about 450 cc. hydrochloric acid (saturated), 10 cc. Rhodineacid inhibitor, and 1350 cc. distilled water. It has been foundpreferable to leave this particular solution in the envelope withagitation for about 1 /2 to 2 minutes, and preferably for about 1%minutes. In this amount of time, said solution dissolves all the oxidebut, at the same time, does not etch the metal to any considerableextent. It will be obvious that the exact composition and concentrationof the chemical solvent and the period for which it is left in theenvelope will depend on the composition of the electrodes and othermetal parts, on the precise structure of the tube, and on the amountsand types of oxides and other impurities present. An essential featureis that the solvent be capable of removing the oxides present bydissolving or loosening and it is highly desirable that it be such as tonot rapidly attack the metal surfaces. As indicated above, where themetal parts are iron or iron alloys, a dilute solution of hydrochloricacid containing a small portion of a suitable acid inhibitor such as isspecified above is desirable.

After the chemical solvent containing the oxides and other impurities isdrained, the envelope is thoroughly rinsed with distilled wter to removethe last traces of the acid and rinsed with acetone, alcohol, or thelike to remove the water. Then one of the exhaust stems is againstoppered and vacuum applied through the other to thoroughly dry theinterior surfaces.

After drying, a wire feeler gauge may be inserted through one of theexhaust stems to accurately measure the gap. between the electrodesurfaces 18 and 20. In this regard, it is advantageous to locate theexhaust stems in alignment with the gap so as to facilitate suchmeasurement. It will be noted that this accurate measurement of thespark discharge gap is taken only after the chemical cleaning operation.for the reason that the gap may significantly increase in size by theremoval of the metal oxide layer which may be of considerable thickness,depending upon the nature of the metals and previous processing stepsused. The importance of this accurate spark gap measurement at thisstage of the process is that tubes having a gap size outside the allowedtolerances may be discarded prior to further processing steps. Anotheradvantage is that such measurement indicates the amount of inert gaspressure which will be required for the tube to impartthe desiredelectrical characteristics.

After the gap measurement is completed, one of the two exhaust stems issealed off with a high temperature gas flame or the like, and theenvelope is again evacuated to remove any water vapor which might havecondensed on the interior surfaces during the exhaust stem sealingoperation. Then, a small quantity of a solution of radioactive materialis introduced into the tube through the stillopen exhaust stem and isallowed to flow over interior surfaces, and particularly the interiorsurface of the glass member 12. About three drops of a solution ofradium bromide containing about 500 micrograms of radium bromide in 50cc. of alcohol may advantageously be used. Introduction of this solutioninto the tube may be by means of a medicine dropper, for example. Aftersuch solution has coated the interior surfaces, the tube is againevacuated, this time to remove the alcohol which is associated with theradium bromide as well as any water vapor which may be present. Then tofurther remove occluded gases, vapors and the like, the electrodes areheated to approximately 750 C. in an induction furnace or the like forabout 30 seconds while vacuum is applied. The tube is allowed to coolwhile the vacuum pumping continues and when cold may be transferred to ahigh vacuum system (diffusion pump) where it is first tested for leaks.For

' discharge occurs.

all vacuum operations up to this point, a mechanical vacuum pump isadequate; however, for this last-mentioned testing step as well as forall subsequent vacuum operations, it is preferable to use a difiusionpump high vacuum system.

After testing for leaks, the tube is placed, while high vacuum pumpingcontinues, in a baking furnace maintained at about 400 C. for aboutone-quarter hour to degas the envelope and to lessen the thermal shockon the tube in preparation for the next processing step. This nextprocessing step consists of admitting into the tube While it is stillhot about 35 mm. of spectroscopically pure hydrogen and applying about1600 volts A. C. across the electrodes with 4000 ohms in series. Thetube electrodes are thus heated slowly and held at a temperature of 750C. for about 2 minutes. At the end of the 2 minutes, the tube is pumpedhard to remove the last traces of hydrogen plus any water which mighthave formed. During this step, the last traces of oxide which might havebeen present in the tube are reduced, the oxygen combining with thehydrogen and being removed as water. Not only are the oxides which mighthave remained after the chemical treatment removed in this manner, butalso any oxides which might have formed during operations subsequent tothe chemical treatment such, for example, as during the sealing off ofthe first exhaust stem and during the addition of the radium bromide.

To further assure the complete removal of the last minute quantities ofoxides in the tube, it is desirable to repeat the last-mentioned processstep at least once and preferably twice. Then the electrodes are heatedto about 75 C. by an induction furnace for about 1%. minutes, the tubebeing pumped hard to remove any absorbed or adsorbed gas from theelectrodes. Then the tube is baked for about 15 minutes in a furnace at400 C., hard pumping continuing at all times during this entire period.This step further assures the removal of any gases which might haveremained in or on the glass envelope. Then the tube is allowed to cool,all the while under high vacuum, and when cold, the degree of vacuum ismeasured with a McCleod gauge or the like. When the tube shows a vacuumof .000005 mm. of mercury or better, the tube is filled with a rare gasmixture to a pressure necessary to attain the desired electricalcharacteristics and then the remaining exhaust stem is sealed closed bya gas flame or the like. For the type of tube hereinbefore described andshown in more detail in U. S. Letters Patent No. 2,478,119, I prefer touse a gas mixture consisting of about 18% neon, 54% argon, and 28%helium, all spectroscopically pure. To facilitate sealing off theexhaust stem, it is highly desirable that the inert gas pressure notsubstantially exceed about mm. below atmospheric pressure.

Since it is necessary that the finished tube discharge at a certainpredetermined voltage, it is highly advantageous to fix the voltagerequired for discharge by adjusting the pressure of the inert gas priorto sealing off the exhaust stem. In this manner a great number of tubeswhich would otherwise have to be rejected because of an inaccurate gapsize may be used just as advantageously as other tubes having a moreaccurate gap size. For example, in one type of tube heretofore describedspecifica tions call for a gap of .041" and a discharge voltage of from680 to 730 peak volts. With a gap of this exact size, 73 centimetersmercury pressure of the above-listed inert gas composition is requiredto provide the specified discharge voltage. However, in practice it isdiflicult if not impossible to control the gap size to any closer thanfrom .036" to .043". In accordance with the present invention, apredetermined voltage the same or close to that for which the tube isdesigned is applied across the electrodes and the gas pressure is variedup or down until At this point, the exhaust stem is sealed off byheating it to its softening temperature and then pinching. Such sealingis aided by the fact that the internal pressure is lower thanatmospheric and thus, when the glass is softened, the outside pressureitself exerts a sealing action.

As mentioned above, it is difficult to seal a tube in which the inertgas pressure is greater than 5 mm. less than atmospheric. When the gapsize is such as to require a gas pressure which exceeds this, the tubemay be cooled a calculated number of degrees by packing with dry ice orthe like and sealing while so cooled to substantially below roomtemperature. In this manner, it is possible to obtain the required gaspressure. The pressure and voltage required at the reduced temperatureto impart the required discharge voltage at room temperature may, ofcourse, be predetermined by calculation. Because the pressure Within anyclosed system varies directly with temperature, it is possible, in thismanner, to utilize tubes which might otherwise have to be discardedbecause of difficulties in sealing the exhaust stem.

Tubes manufactured in accordance with the invention may require abreak-in period during which the voltage required to cause dischargerapidly increases, then decreases and finally becomes constant. However,such changes in the voltage required are relatively constant for anygiven type of tube and thus, it is only necessary to seal at apredetermined voltage and run the tubes through a break-in period toimpart the desired constant discharge voltage.

It is to be understood that, although the invention has been describedwith specific reference to particular embodiments thereof, it is not tobe so limited since changes and alterations therein may be made whichare within the full intended scope of this invention as defined by theappended claims.

What is claimed is:

1. A process for manufacturing a gaseous discharge tube, including thesteps of sealing a pair of electrodes consisting of a mixture of ironand barium oxide within a glass envelope to form the tube, introducinginto said tube an aqueous solution of hydrochloric acid, agitating saidacid within said tube for a suflicient period to remove oxide and otherimpurities from the interior surfaces of said tube, removing said acid,drying the interior of said tube, introducing radioactive materialdissolved in a liquid to the interior of said tube, evacuating said tubeto remove said liquid, again evacuating said tube while heated to about750 C. to remove vapors and occluded gases, introducing hydrogen to theinterior of said tube, heating said tube to cause said hydrogen to reactwith any oxide present on the interior surfaces of said tube, againevacuating said tube, introducing rare gas into said tube until apressure required to permit discharge at a predetermined voltage isattained and then sealing said tube.

2. A process for manufacturing a gaseous discharge tube, including thesteps of introducing into said tube a liquid chemical solvent for oxideand other impurities, agitating said solvent Within said tube for asufiicient period to remove the oxide and other impurities from theinterior surfaces of said tube, removing said solvent, drying theinterior of said tube, introducing radioactive material dis solved in aliquid to the interior of said tube, evacuating said tube to remove saidliquid, again evacuating said tube While heated to about 750 C. toremove occluded gases, introducing hydrogen to the interior of saidtube, heating said tube to cause said hydrogen to react with any oxidepresent on the interior surfaces of said tube, again evacuating saidtube, introducing rare gas into said tube until a pressure required topermit discharge at a predetermined voltage it attained and then sealingsaid tube.

3. A process for manufacturing a gaseous discharge tube including thesteps of introducing into said tube a liquid chemical solvent for oxideand other impurities, agitating said solvent within said tube for asufiicient period to remove the oxide and other impurities from theinterior surfaces of said tube, removing said solvent, in-

troducing radioactive material to the interior of said tube, introducinghydrogen to the interior of said tube, 'heating said tube to cause saidhydrogen to react with any oxide present on the interior surfaces ofsaid tube, again evacuating said tube, introducing rare gas into saidtube and then sealing said tube.

4. A process for manufacturing a gaseous discharge tube, including thesteps of introducing into said tube a liquid chemical solvent for oxideand other impurities, allowing said solvent to remain in said tube for asufiicient period to remove the oxide and other impurities from theinterior surfaces of said tube, removing said solvent, evacuating saidtube to remove vapors and occluded gases, introducing hydrogen to theinterior of said tube, heating said tube to cause said hydrogen to reactwith any oxide present on the interior surfaces of said tube, againevacuating said tube, introducing rare gas into said tube until apressure required to permit discharge at a predetermined voltage isattained and then sealing said tube.

5. A proces for manufacturing a gaseous discharge tube, including thesteps of introducing into said tube a liquid chemical solvent for oxideand other impurities, allowing said solvent to remain in said tube for asutficient period to remove the oxide and other impurities from theinterior surfaces of said tube, removing said solvent, introducingradioactive material dissolved in a liquid to the interior of said tube,removing said liquid, introducing hydrogen to the interior of said tube,heating said tube to cause said hydrogen to react with any oxide presenton the interior surfaces of said tube, evacuating said tube, introducingrare gas into said tube and then sealing said tube.

6. A process for manufacturing a gaseous discharge tube, including thesteps of introducing into said :tube a liquid chemical solvent for oxideand other impurities, allowing said solvent to remain in said tube for asufiicient period to remove the oxide and other impurities from theinterior surfaces of said tube, removing said solvent, introducinghydrogen to the .interior of said tube, heating said tube to cause saidhydrogen to react with any oxide remaining on the interior surfaces ofsaid tube, evacuating said tube, introducing rare gas into said tubeuntil a pressure required to permit discharge at a predetermined voltageis attained and then sealing said tube.

7. A process for removing oxide and other impurities from the interiorsurfaces of a gaseous discharge tube during the manufacture thereof,including the steps of introducing into said tube a liquid chemicalsolvent for the oxide and other impurities, allowing said solvent toremain in said tube for a sufficient period to remove the oxide andother impurities from the interior surfaces of said tube, removing saidsolvent, introducing hydrogen to the interior of said tube, heating saidtube to cause said .hydrogen to react with any oxide remaining on theinterior surfaces of said tube and evacuating said tube.

8. A process for removing oxide and other impurities from the interiorsurfaces of a gaseous discharge tube during the manufacture thereofincluding the steps of introducing into said tube an acid solvent fromthe oxide and other impurities, agitating said solvent within said tubefor a sufficient period to remove the oxide and other impurities fromthe interior surfaces of said tube, removing said solvent, evacuatingsaid tube while heated to remove vapors and occluded gases, introducinghydrogen to the interior of said tube, heating said tube to cause saidhydrogen to react with any oxide remaining on the interior surfaceso'fsaid tube, and again evacuating said tube.

9. A process for manufacturing a gaseous discharge tube, including thesteps of sealing a plurality of metal electrodes within a glass envelopeto form the tube, introducing into said tube a liquid chemical solventfor oxide and .other impurities, allowing said solvent to remain in saidtube for a sufficient period to remove the oxide and other impuritiesfrom the interior surfaces of said tube, removing said solvent andsubsequently treating the interior of said tube with hydrogen to reducethe remaining oxide impurities in said tube.

10. A process for manufacturing a gaseous discharge tube, including thesteps of sealing a plurality of ferrous metal electrodes within a glassenvelope to form the tube, introducing into said tube an aqueoussolution of hydrochloric acid, agitating said acid within said tube fora sufiicient period to remove oxide and other impurities from theinterior surfaces of said tube, removing said acid, drying the interiorof said tube, introducing hydrogen to the interior of said tube, heatingsaid tube to cause said hydrogen to'react with any oxide remaining onthe interior surfaces of said tube, evacuating said tube, introducingrare gas into said .tube until a pressure required to permit dischargeat a predetermined voltage is attained and then sealing said tube.

11. In a process for manufacturing a gas filled electronic device of thetype having a glass envelope provided with an exhaust stem, the steps ofevacuating said envelope, cooling said envelope to substantially belowroom temperature, filling said envelope with a gas to a pressure notgreater than 5 mm. mercury below atmospheric pressure, sealing theexhaust stem by the application of heat thereto to soften the glass andthen allowing said envelope to warm to room temperature.

References Cited in the file of this patent UNITED STATES PATENTS1,165,338 Moench Dec. 21, 1915 2,362,510 Stutsman Nov. 14, 19442,560,273 Brigant et a1. July 10, 1951 FOREIGN PATENTS 608,268 GreatBritain Sept. 13, 1948 UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 2,800,383 July 23, 1957 Ralph H. Mitchel It ishereby certified that error appears in the printed specification f theabove numbered patent requiring correction and that the said Let oersatent should read as corrected below.

Column 2, line 15, for "mixtured" read mixture column 3, inc 31, for"ion" read iron line 43, for "sofetn" read soften; olumn 4, line 25, for"portion" read proportion line 29, for wiser" read water column 6, line69, for "voltage it" read voltage is column 7, line 21, for 'proces"read process olumn 8, line 53, list of references cited, under "UNITEDSTATES PATENTS", or "Brigant" read Briganti Signed and sealed this 1stday of October 1:957

SEAL) :test:

LARL H, AXLINE ROBERT C. WATSON testing Officer Conmisaioner of Patents

